Application of the orbital deletion procedure (ODP) to planar carbocations

An orbital deletion procedure (ODP), which inactivates a critical pi orbital and therefore interrupts cyclic conjugation, and the nucleus independent chemical shifts (NICS) have been employed to evaluate the aromaticity of the tropylium, cyclopropenyl and benzyl cations, as well as the antiaromaticity of the singlet cyclopentadienyl cation, compared with their reference cations: 1,3,5-heptatrienyl, allyl, 1,3,5-hexatrienyl-3-carbinyl and 1,3,5-pentadienyl. The aromatic stabilization energies, defined as the difference in the delocalization energies (deduced from ODP) between the cyclic and the acyclic reference cation, are in general agreement with the values from appropriate isodesmic equations. (C) 1998 Published by Elsevier Science B.V. All rights reserved.